In recent years, in the field of radio communication, especially in mobile communication, a variety of information such as images and data in addition to speech is transmitted. The demand for higher speed transmission is expected to further increase in the future, and, to perform high-speed transmission, a radio transmission scheme that utilizes limited frequency resources more effectively and achieves high transmission efficiency is in demand.
OFDM (Orthogonal Frequency Division Multiplexing) is one of radio transmission techniques, to meet these demands. OFDM is one of multicarrier communication techniques, whereby data is transmitted in parallel using a large number of subcarriers, and it is known that OFDM has features of providing high frequency efficiency and reducing inter-symbol interference in a multipath environment and is effective to improve transmission efficiency.
Studies are being conducted to perform frequency scheduling transmission and frequency diversity transmission using this OFDM on the downlink, when a radio communication base station apparatus (hereinafter simply “base station”) frequency-domain-multiplexes on a plurality of subcarriers data for a plurality of radio communication mobile station apparatuses (hereinafter simply “mobile stations”).
In frequency scheduling transmission, the base station adaptively allocates subcarriers for mobile stations, based on the received quality of each frequency band in each mobile station, so that it is possible to obtain a maximum multi-user diversity effect. This frequency scheduling transmission is mainly suitable for mobile stations moving at low speed. Meanwhile, to perform frequency scheduling transmission, feedback of received quality information from the mobile stations to the base station is necessary, and therefore, frequency scheduling transmission is not suitable for the mobile station moving at high speed. Further, frequency scheduling transmission is usually performed on every resource block grouping a plurality of neighboring subcarriers. That is, in frequency scheduling transmission, data for mobile stations is mapped to subcarriers collectively per resource block, that is, localized mapping is performed, and therefore, not much high frequency diversity effect is obtained.
By contrast with this, in frequency diversity transmission, data for mobile stations is mapped to subcarriers in a distributed manner over the entire band, that is, distributed mapping is performed, so that a high frequency diversity effect can be obtained. Further, frequency diversity transmission does not require received quality information from the mobile stations, and therefore, frequency diversity transmission is a useful scheme where frequency scheduling transmission is difficult to apply. On the other hand, frequency diversity transmission is performed regardless of received quality in the mobile stations, and therefore it is not possible to obtain multi-user diversity effect such as in frequency scheduling transmission.
Further, to perform frequency scheduling transmission, the base station transmits, to the mobile stations of data transmission destinations per subframe, control signals formed with mobile station IDs (i.e. user IDs), resource block numbers, modulation and coding schemes (MCSs) for data channels, types of control information and so on, at the beginning of each subframe, prior to data transmission. Further, these control signals are transmitted in SCCHs (Shared Control Channel). SCCHs are provided in the number of mobile stations to which data is transmitted in the subframe, and the number of mobile stations per subframe is defined by, for example, frequency bandwidths available in the communication system. That is, at the beginning of each subframe, SCCHs in the same number as data channels in the subframe, is multiplexed over the same time.
Then, studies are underway to adopt frequency scheduling transmission and frequency diversity transmission to the SCCHs recently (see Non-patent Document 1). That is, studies are conducted to perform localized mapping and distributed mapping on a control signal transmitted in the SCCHs. In this case, control signals transmitted in the SCCHs include mapping methods of the control signals. Then, mobile stations receiving these control signals identify the content of these control signals by comparing these received control signals against all patterns that combinations of the mapping methods and the information contents can adopt one to one. That is, the mobile station performs blind detection of the control signal in the SCCHs.    Non-patent Document 1: 3GPP RAN WG1 Meeting document, R1-063177